A luminescent material absorbs radiation from one region of the electromagnetic (EM) spectrum and emits radiation in another region of the electromagnetic spectrum, the emission generally being lower in energy than the absorption (i.e., Stokes shifted). A luminescent material in powder form is commonly called a phosphor, while a luminescent material in the form of a transparent solid body is commonly called a scintillator.
Two broad classes of luminescent materials are generally recognized. These are self-activated luminescent materials and impurity-activated luminescent materials.
A self-activated luminescent material is one in which the pure crystalline host material, upon absorption of a high energy photon, elevates electrons to an excited state from which they return to a lower energy state by emitting a photon. Self-activated luminescent materials normally have a broad spectrum emission pattern because of the relatively wide range of energies that the electron may have in either the excited or the lower energy states. Thus, any given excited electron may emit a fairly wide range of energy during its transition from its excited state to its lower energy state, depending on the particular energies it has before and after its emissive transition.
An impurity-activated luminescent material is normally one in which a non-luminescent host material has been modified by inclusion of an activator species (i.e., dopant), which is typically present in the host material in a relatively low concentration, such as in the range from about 200 parts per million (ppm) to 1 part per thousand. However, some materials require several mole or atomic percent of activator ions for optimized light output. With an impurity-activated luminescent material, the activator ions may either absorb the incident photons directly, or the lattice may absorb the incident photons and transfer the absorbed photon energy to the activator ions.
Luminescent phosphor materials find widespread application in fluorescent lighting applications, where UV emission from mercury (Hg) gas is absorbed by the phosphor material and emitted as visible light. Other applications for such phosphor materials include the tuning of light emitted by light emitting diodes (LEDs). Such tuning can allow for the generation of white light using a single type of LED.
The conversion of a single ultraviolet (UV) photon into two visible (vis) photons with the result that the quantum efficiency of luminescence exceeds unity is termed quantum splitting. Quantum splitting materials are very desirable for use as phosphors for lighting applications, such as fluorescent lamps. A suitable quantum splitting phosphor can, in principle, produce a significantly brighter fluorescent light source due to higher overall luminous output because it can convert to visible light that part of the UV radiation that is not absorbed efficiently by traditional phosphors that are currently used in commercial fluorescent lamps. Quantum splitting has been demonstrated previously in fluoride- and oxide-based materials. A material comprising 0.1% Pr3+ in a matrix of YF3 has been shown to generate more than one visible photon for every absorbed UV photon when excited with radiation having a wavelength of 185 nm. The measured quantum efficiency of this material was 140%, and thus greatly exceeded unity. However, fluoride-based compounds do not have sufficient stability to permit their use as phosphors in fluorescent lamps because they are known to react with mercury vapor that is used in such lamps to provide the UV radiation. Such reaction can form materials that do not exhibit quantum splitting. In addition, producing fluoride-based materials presents a great practical challenge because it involves the use of large quantities of highly reactive and toxic fluorine-based materials.
Lumen maintenance in halophosphate phosphors has been improved by the addition of cadmium (Cd). However, the high toxicity associated with Cd-containing materials has led to legislation precluding the use of such phosphors. The maintenance and lamp efficacy of phosphors such as Zn2SiO4:Mn2+ has been improved by coating with non-emitting, high stability, wide-band-gap materials, such as Al2O3 and Y2O3.
More recently, oxide-based quantum splitting phosphors have been developed that overcome the disadvantages of fluoride based materials in fluorescent lighting applications. See, for example, A. M. Srivastava et al., “Luminescence of Pr3+ in SrAl12O19: Observation of two photon luminescence in oxide lattice,” J. Luminescence, 1997, 71, pp. 285-290; and commonly-assigned U.S. Pat. Nos. 5,571,451 and 6,613,248. Such materials are generally aluminates or borates doped with Pr3+. A particularly good quantum splitting phosphor material is a strontium magnesium aluminate activated with Pr3+ and charge compensated with Mg2+. This phosphor is abbreviated: SrAl12O19:Pr,Mg.
While the above-mentioned oxide-based quantum-splitting phosphors overcome many of the limitations of halide-based quantum splitting phosphors, they generally have particle sizes large enough to scatter 254 nanometer (nm) radiation and would thereby decrease the efficiency of fluorescent lamps in which they are used.
In light of the above, a phosphor material with reduced scattering, due to smaller crystallite size (e.g., nanocrystals) and thinner coatings, would be of great benefit, especially if it were a quantum splitting phosphor.